Betelgeuse Dimmed So Dramatically in 2019 That Astronomers Thought It Might Be About to Explode
The red supergiant star in Orion's shoulder lost two-thirds of its brightness in a matter of months. The explanation, when it came, was stranger than a supernova.
Betelgeuse is one of the largest stars visible to the naked eye — a red supergiant in the constellation Orion, roughly 700 light-years from Earth, with a diameter so vast that if it replaced the Sun at the centre of our solar system, its surface would extend beyond the orbit of Jupiter. It is also a star that astronomers have long known is approaching the end of its life. When it dies, it will die as a supernova — an explosion visible from Earth in daylight. The question has always been when.
Between October 2019 and February 2020, Betelgeuse dimmed dramatically. Its visual magnitude dropped from approximately 0.5 to 1.6 — a reduction of about two-thirds in apparent brightness. For a star that had been one of the ten brightest in the night sky for as long as humans had been observing it, this was extraordinary. The astronomical community took notice. The popular press ran headlines about an imminent supernova. The question of whether Betelgeuse was about to explode became, briefly, a mainstream news story.
It did not explode. By April 2020, the star had returned to near-normal brightness. The explanation, when it emerged from subsequent analysis, was both mundane and remarkable: Betelgeuse had sneezed. The star had ejected a large mass of material from its surface — a surface mass ejection, analogous to but far larger than the coronal mass ejections produced by our own Sun. This ejected material cooled as it moved away from the star and condensed into dust, which temporarily obscured a portion of the star's surface from our line of sight.
The surface mass ejection was unprecedented in recorded astronomical observation. The amount of material ejected was estimated at several times the mass of the Moon, expelled in a single event. The dust cloud it produced was large enough to be directly imaged by the Very Large Telescope in Chile — one of the first times a feature on the surface of a star other than the Sun had been directly observed.
The Great Dimming did not indicate an imminent supernova. But it did reveal that Betelgeuse is considerably more volatile than previously understood, and that the processes leading to a supernova may be more complex and more dramatic than the existing models suggest. The star has continued to behave unusually since 2020, with brightness variations that have not fully returned to their pre-dimming patterns.
Betelgeuse will explode. The current best estimates suggest this will happen sometime in the next 100,000 years — a timeframe that is, on astronomical scales, essentially imminent. When it does, it will be visible from Earth in daylight for weeks. It will be the brightest object in the night sky for months. It will be, by a considerable margin, the most dramatic astronomical event in recorded human history. The Great Dimming was a reminder that we are watching a star in its final act, and that the final act has already begun.